Can I upgrade or downgrade my cabin with Tallink Silja before departure? Say I booked a cabin for two and now two more friends want to join, can I change to a premium cabin for four? Or will the other two friends have to make a new reservation?
In addition to Daniils answer: while on board, you can always contact the customer service desk. Even if cabins are shown as sold out prior to departure, there may always be some no-shows, and after departure, they may be willing to upgrade you. When we took the ferry from Savona to Barcelona, cabins were sold out at booking time, but we still managed to upgrade to one after the ferry had departed. This is at the discretion of the crew and it never hurts to ask, even if the possibility is not spelt out online or the website shows cabins as fully booked.
According to their Terms and Conditions:
3. TICKET CHANGING
(1) It is allowed to change the direction and/or itinerary set on the ticket, the date or time of departure, cabin class, vehicle and on-board service, and any changes to the above-mentioned services may change the fare to be paid.
If you scroll down on that page there are specific time frames in which you can change your ticket depending on the ferry route.
To add passengers you might need contact them:
Phone number: +49(0)40 547 541 222